a) Field of the Invention
The invention relates to an adjustable steering column for a motor vehicle with a fixing device, in the opened state of which the steering column is adjustable in at least one displacement direction and in the closed state a set position of the steering column is secured in place. The adjustable steering column comprises at least one securement part, which has at least one toothing with teeth, and at least one counter-securement part, which has at least one counter toothing with teeth, wherein in the closed state of the fixing device teeth of the securement part mesh with teeth of the counter-securement part and during the opening of the fixing device the counter-securement part is withdrawn from the securement part in one opening direction and in the opened state of the fixing device the teeth of the counter-securement part are out of engagement with the teeth of the securement part.
b) Description of Related Prior Art
Adjustable steering columns are known in different embodiments. For adapting to the seating position of the driver, such adjustable steering columns can be adjusted in length (=in the axial direction) and/or in their angular position (=in their inclination), wherein in passenger cars a height adjustment results by adjusting the angular position.
Fixing devices serve to secure in place the set position of the steering column. The fixing devices act through positive locking and/or frictional locking. These fixing devices must, on the one hand, be able to absorb the forces occurring in normal operation in order to maintain the set position of the steering column; on the other hand, in the event of a crash the uncontrolled displacement of the steering column must be prevented in order, for example, for the correct position of an airbag disposed in the steering wheel to be maintained and/or the energy upon the impact of the driver onto the steering wheel to be dissipated under control. Fixing devices acting under frictional locking with disk lamellae cooperating in the manner of a disk lamella pack are disclosed, for example, in EP 0 802 104 B1 or U.S. Pat. No. 6,581,965 B2.
Further known are adjustable steering columns in which the fixing device comprises toothed parts which, in the closed state of the fixing device, engage into one another and consequently block the length displacement and/or the angular displacement of the steering column. Such a steering column is disclosed, for example, in EP 0 836 981 B1, EP 0 755 842 B1, DE 198 39 496 A1, U.S. Pat. No. 5,787,759 A and DE 101 30 587 A1.
Adjustable steering columns have already been proposed in which different action principles are applied in different displacement directions, for example a friction-locking securement of the length displacement and a positive-locking securement of the angular displacement.
To save weight, adjustable steering columns should as much as possible be light-weight, leading to a certain elasticity of the fixing device also. However, the fixing device must have adequate stability in order to fix the set position of the steering column even under high acting forces, such as forces that occur in the event of a crash. In a fixing device acting under frictional locking in one displacement direction herein sufficiently high press-on forces must be applied for the mutual pressing of the friction faces on one another. Due to the oblique tooth flanks in toothings, forces also act in the direction of a distancing of the parts comprising the toothings, which must be absorbed by the fixing device such that the entire construction must be implemented relatively solidly.
In fixing devices acting under positive locking through a tooth engagement of toothings, a problem is encountered that during the closing of the fixing device the tooth combs of the toothings opposing one another can impact upon one another (“tooth-on-tooth” position) wherein the fixing device cannot be closed properly. To decrease this problem, the tooth combs are conventionally formed as acutely as possible, i.e., the side flanks of the teeth which slope toward each other when viewed in cross section are connected with one another in the region of the tooth comb by as small a radius as possible. Furthermore, various devices have already been proposed, by means of which the impacting of the tooth tips during the closing of the fixing device is to be prevented. In the device of DE 198 39 496 A1, which is configured for this purpose, one of the securement parts including the toothings is bearing-supported swivellably about an axis which is at right angles to the displacement direction and at right angles to the opening direction of the two securement parts. In the opened state the swivellably bearing-supported securement part, through swivelling about this transverse axis, is positioned obliquely to the other securement part. Such an implementation is also disclosed in U.S. Pat. No. 5,787,759 A. The teeth of the toothings are here formed in the manner of saw teeth, i.e., one of the side flanks of each tooth is at an angle to the opening direction, the other side flank is parallel to the opening direction (for the swivellably bearing-supported securement part this applies to the closed state of the fixing device).
A further such fixing device with a securement part swivellable about a transverse axis is disclosed in DE 101 30 587 A1, wherein the teeth of the toothings of the securement parts which can be brought into engagement with one another, being again formed in the manner of saw teeth.